In the microelectronics industry, a die carrying an integrated circuit is commonly mounted on a package carrier, such as a substrate, a circuit board or a leadframe, that provides electrical connections from the die to the exterior of the package. In one such packaging arrangement called flip chip mounting, the die includes an area array of electrically conductive contacts, known as bond pads, that are electrically connected to corresponding area array of electrically-conductive contacts on the package carrier, known as solder balls or bumps. Typically, the solder bumps are registered with the bond pads and a reflow process is applied to create electrical connections in the form of solder joints between the die and the package carrier. The process of flip chip mounting results in a space or gap between the die and the package carrier.
The die and the package carrier are usually formed of different materials having mismatched coefficients of thermal expansion. As a result, the die and the package carrier experience significantly different dimension changes when heated that creates significant thermally-induced stresses in the electrical connections between the die and the package carrier. If uncompensated, the disparity in thermal expansion can result in degradation in the performance of the die, damage to the solder joints, or package failure. As the size of the die increases, the effect of a mismatch in the coefficient of thermal expansion between the die and the substrate becomes more pronounced. In stacked die packages, the mismatch in the coefficients of thermal expansion between the die laminate and the package may be even greater than in single die packages. The failure mechanism in stacked die packages may shift from solder joint damage to die damage.
To improve the reliability of the electrical connections in flip chip package assemblies, it is common in the microelectronics industry to fill the gap between the die and the package carrier with an encapsulant material. Underfilling with encapsulant material increases the fatigue life of the package and improves the reliability of the electrical connections by reducing the stress experienced by the electrical connections during thermal cycling or when the die and the package carrier have a significant temperature differential. The encapsulant material also isolates the electrical connections from exposure to the ambient environment by hermetically sealing the gap and lends mechanical strength to the package assembly for resisting mechanical shock and bending. The encapsulant material further provides a conductive path that removes heat from the die and that operates to reduce any temperature differential between the die and substrate. As a result, underfilling with encapsulant material significantly increases the lifetime of the assembled package.
Various conventional underfilling methods are used to introduce the encapsulant material into the gap between the die and the substrate. One conventional method relies on surface tension wetting or capillary action to induce movement of a low-viscosity encapsulant material with strong wetting characteristics from a side edge into the gap. According to this method, encapsulant material is dispensed along side edges of the die, and capillary forces operate to attract the encapsulant material into the gap. Typically, the viscosity of the encapsulant material is reduced and the flow rate increased by pre-heating the substrate in the vicinity of the die before the encapsulant material is dispensed onto the substrate. Put another way, the heat assists encapsulant adhesive to flow out more freely and wick into small cavities on the substrate. The underfill material is subsequently cured after the electrical connections have been fully encapsulated.
To this end, many conventional techniques involve positioning the substrate onto a heated block surface. This method of heating by contact, however, is effective only where the substrate surface is flat, e.g., has not been populated with mounted components. In all other cases, it is necessary to use a non-contact heating method, such as by blowing heated air onto the substrate. More particularly, a typical non-contact, or air impingement system includes a heated aluminum block. The top surface of the block, upon which the substrate rests, has formed apertures that allow for the through passage of air to the substrate. Resistance heating elements contained within the block heat the air as it is blown from an air plenum positioned below the heated block to the substrate.
While generally effective in facilitating encapsulant flow, the heavy aluminum construction of such conventional blocks imputes a high thermal mass, resulting in relatively slow warm-up and cool down times. The heavy construction of the blocks often makes them cumbersome to position and complicates accommodating any special unusual or customized product requirements.
Moreover, the mechanisms responsible for providing the airflow through the block can limit the efficiency of an underfilling operation. A conventional non-contact system incorporates a manually adjustable flow valve to control the airflow through the plenum/block towards the substrate. This valve is often incorrectly set by an operator, resulting in improperly heated substrates. Also, air is typically pumped continuously regardless of whether a part is actually in position. This can result in wasted thermal energy and higher rates of consumption of air and electricity, in addition to faulty production.
There consequently exists a need for an improved underfilling system and associated process.